Method and apparatus for microwave treatment of dielectric films

ABSTRACT

An apparatus for thermal treatment of dielectric films on substrates comprises: a microwave applicator cavity and microwave power source; a workpiece to be heated in the cavity, comprising a porous coating on a selected substrate; and, a means of introducing a controlled amount of a polar species into the porous coating immediately before heating by the microwave power. The interaction of the polar species with the microwaves enhances the efficiency of the process, to shorten process time and reduce thermal budget. A related method comprises the steps of: depositing a porous film on a substrate; soft baking the film to a selected state of dryness; introducing a controlled amount of a polar species into the soft baked film; and, applying microwave energy to heat the film via interaction with the polar species.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Ser. No.61/854,556, filed Apr. 26, 2013 by the present inventor, the entiredisclosure of which is incorporated herein by reference. The applicationis further related to the following utility applications, all filed oneven date herewith: Method and Apparatus for Microwave Treatment ofDielectric Films (Docket No. Lambda 17.2), and Method and Apparatus forMicrowave Treatment of Dielectric Films (Docket No. Lambda 17.3), theentire disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to apparatus and method for tailored microwaveheat treatment of low dielectric constant (k) organo-silicate glass(OSG) films deposited on substrates for semiconductor devices. Thisincludes the heat treatment for (a) porogen removal as well as (b)restoration of plasma damaged porous low-k dielectrics films.

2. Description of Related Art

As integrated circuit feature sizes continue to shrink, new lowdielectric constant (low-k) materials are needed to address problemswith power consumption, signal propagation delays, and crosstalk betweeninterconnects. One avenue to low-k dielectric films is the introductionof nanometer scale pores to lower the effective dielectric constant,which can therefore replace the dense silicon oxide insulator materials.Various materials and methods have been explored for deposition ofporous organo-silicate glass (OSG) low-k films during the last 10-15years. OSG materials have a silica-like backbone structure with afraction of the Si—O bonds replaced with organic groups such as —CH₃.This reduces overall dielectric constant of the material.

The relative dielectric constant of dense OSG is limited to k valuesgreater than ˜2.7. It is expected that materials with even lowerdielectric constants are needed for future generations of integratedcircuits. To this end, a new class of OSG materials with porousstructures introduced into the dense matrix has been successfullysynthesized with k values as low as 2.0, which could be considered asleading candidates for use as interconnection dielectrics in emergingtechnologies.

The porous OSG films can be prepared by using a self-assembledtechnology to form nano-composite structures with controlled structureand physical properties. The spin-on low-k films are formed bycondensing a hydrolyzed alkylated silica sol in the presence of apolymeric surfactant. This surfactant acts as a template to produce aregular porous structure as the film dries. Upon heat treatment thesurfactant acts as a porogen and evaporates, thereby leaving behind aporous silica network with alkyl groups (e.g. methyl —CH₃), whichpassivate the internal and external film surfaces.

The spin-on porous dielectric films have compositions similar to thepopular plasma-enhanced chemical vapor deposition (PECVD) filmmaterials. For the PECVD approach, a sacrificial porogen is added to thegaseous mix, which on heat treatment or processing is removed from thefilm and thus the pores are formed within the film. The porogen loadingcan be changed to alter the film porosity in the range of 7-45%. Theheat treatment is usually performed at ˜400° C. or higher to completelyremove the porogen, minimize the dielectric value k and simultaneouslyenhance the mechanical strength of the film.

However, successful implementation of these porous dielectricsstructures poses numerous challenges. As compared to their densecounterparts, porous dielectrics are expected to have reduced cohesivestrength and poorer adhesion with adjacent layers. They are more proneto the absorption of reactive chemicals during device fabrication, somuch so that water can diffuse quite effectively into film stackscontaining dielectric layer, even though the dielectric materials areusually hydrophobic. The moisture uptake could vary from ˜0.54 wt. % to1.7 wt. % as the porosity is raised to 40 vol. %. The ingress of waterinto the dielectric negatively impacts both the mechanical integrity aswell as electrical performance of the devices. Some isotope tracerdiffusion and secondary ion mass spectroscopy experiments have revealedthat water diffuses predominantly along the interface and not throughthe porous films. This result was attributed to the hydrophobic natureof the dielectric material and the hydrophilic character of theinterfaces—hence the degradation of the interfacial adhesion.

Another challenge for these porous OSG low-k dielectrics is that theyare susceptible to damage induced during etching and cleaning process,which degrades the dielectric and electrical properties and surfaceroughness. The plasma exposure breaks the weakly bonded organic terminalgroups from the silica backbone while simultaneously densifying theporous silica-like damaged skin layer. Much effort is being dedicated,in the first place, to minimize the plasma damage. As a second choice,post-patterning remedies intended for dielectric restoration are beingexplored as well. Usually, trialkyl-substituted disilazanes orchlorosilanes such as hexamethyldisilane (HMDS) andtrimethylchlorosilane (TMCS) or dimethylaminotrimethylsilane (DMATMS)and so on, are in use for dielectric restoration.

Objects and Advantages

Objects of the present invention include the following: providing asystem for infiltrating a porous film with a solvent immediately beforemicrowave processing; providing a system for in-situ heating of asolvent-containing porous film using microwave energy; providing anapparatus for delivering controlled amounts of polar solvents to porousdielectric films or soft baked polymeric films to enhance the couplingwith microwaves and enhance the reaction in the film material; providinga method to uniformly heat treat solvent-containing films deposited onsubstrates for selected applications; and, providing a method forreducing the thermal budget for processing low-k dielectric films. Theseand other objects and advantages of the invention will become apparentfrom consideration of the following specification, read in conjunctionwith the drawings.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an apparatus for thermaltreatment of dielectric films on substrates comprises:

a microwave applicator cavity;

a microwave power source to deliver power to the applicator cavity;

a workpiece to be heated in said cavity, the workpiece comprising aporous coating on a selected substrate; and,

a means of introducing a controlled amount of a polar species into theporous coating immediately before heating by the microwave power.

According to another aspect of the invention, a method for processing adielectric film on a substrate comprises the steps of:

depositing a porous film on a substrate;

soft baking the film to a selected state of dryness;

introducing a controlled amount of a polar species into the soft bakedfilm; and,

applying microwave energy to heat treat the film, wherein the treatmentis enhanced by the interaction of microwave energy with the polarspecies.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerconception of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore non-limiting embodimentsillustrated in the drawing figures, wherein like numerals (if they occurin more than one view) designate the same elements. The features in thedrawings are not necessarily drawn to scale.

FIG. 1 illustrates schematically an apparatus that will allow solvent orreagent vapors from a single or multiple sources delivered on to aporous film and in-situ microwave heat treatment of the dielectric filmon the substrate.

FIG. 2 illustrates the schematic of variation in pressure, applicationof microwave and the temperature rise as well as the cyclic naturepossible with microwaves heating.

FIG. 3 illustrates schematically another example of the invention,configured in a cluster tool arrangement.

DETAILED DESCRIPTION OF THE INVENTION

Microwave energy because of its rapid and internal heating mechanism hasbeen one of the very attractive means of heat treatment. For curingprocesses microwaves interact with polar groups of molecules in theorganic materials, enhance their mobility because of the rotationmovement of molecules and hence enhance the cross-linking of monomers orcure of materials. In other semiconductor materials or coatings, themicrowave induced transport of current carriers and possiblepolarization, the anneal processes can be performed in shorter times orat somewhat lower temperatures.

Microwave energy is especially beneficial for porous materials, becauseunlike any other heating technology, it has the ability to interact withthe polar molecules in the pores of the material and rotate and agitatethese polar molecules. Since these porous films are more prone to theabsorption of reactive chemicals, we use it to our advantage. The key isto first intentionally infiltrate polar (vapor) molecules into thepores. Then apply microwave energy to interact with the infiltratedpolar molecules to enhance the desired process, by enhancing themobility of the species taking part in the reaction. This understandingand its use constitutes an important aspect of this invention.

The use of microwave energy could be effectively employed for theporogen removal process by first infiltrating polar solvent into thepores, followed by microwave exposure to rotate or agitate the polarsolvent molecules within the pores, which will assist in the porogenremoval. It can also be used for restoration of the surface from damagecaused by plasma contact during etching and cleaning processes. One ofthe chemicals hexamethyldisilane (HMDS) generally useful forconditioning chromatography columns and deactivating glassware is alsoused for plasma damage restoration and is a very popular chemicalreagent for enhancing synthesis reaction with microwaves. Therefore,HMDS vapors can be infiltrated into the pores of the dielectric materialand followed by microwave exposure. Any polar groups or molecule withinthe pores of the dielectric will be set into rotation thereby providingagitation of HMDS molecules to assist in the restoration of the entiresilica surface area damaged with plasma processes. Anywhere thesemolecules can infiltrate microwave has the ability to agitate them deepwithin the pores and assist in enhancing the reaction. Similarmechanisms will be in play for other reagents TMCS or DMATMS and so on,listed above. Variable Frequency Microwave (VFM) is well suited forprocessing semiconductor materials and thin film coatings used fornumerous electronics, semiconductor wafers, flat panel displays andphotovoltaic applications. The basic VFM approach is well-known andtaught in at least the following U.S. patents, each of which isincorporated herein by reference in its entirety: U.S. Pat. Nos.5,321,222; 5,721,286; 5,961,871; 5,521,360; 5,648,038; and 5,738,915,7,939,456, and 8,021,898. In particular, the continuous sweeping offrequencies over the available bandwidth, as taught in theaforementioned references, reduces the potential for arcing andsubsequent damage. Frequency sweeping is often carried out by selectinga center frequency and then rapidly sweeping the frequency in asubstantially continuous way over some range (typically +/−5% of thecenter frequency, although this range can vary depending on such factorsas the type of microwave source, and the overall size of the cavitycompared to the microwave wavelength). Numerous kinds of wafers withintegrated circuits have been exposed to VFM and it has beendemonstrated that there is no damage to the circuits or theirfunctionality. The use of VFM provides more rapid processing as comparedto conventional heat treating furnaces.

As noted earlier, one objective of the invention is to provide uniformand rapid microwave heat treatment of thin dielectric films on largesubstrates, especially those with metallization that cannot be easilyprocessed with single frequency microwave because of the potential forcharge build, leading to arcing and hence damaging the electronicdevices or circuits. However, on smaller substrates, especially thosewith simple organic coatings and little or no metallic traces, singlefrequency microwaves applies equally well and are thus not excluded fromthis invention. Therefore, microwaves have been frequently used in thisinvention and where ever necessary, the preferred Variable FrequencyMicrowaves (VFM) sources could be used on the equipment instead of fixedfrequency microwave sources.

It is instructive to review the general dynamics of polar molecules whenexposed to microwave energy. As we already know, while heating food withmicrowaves the polar water molecule is set into rotation. With all therotating water molecules adjacent to each other and the friction betweenthese molecules, food is rapidly heated with the penetrating microwaveenergy. However, with stoves and convection ovens, heat has to beconducted through water and the water molecule will simply have lineartranslational movement. When water is at higher temperatures the atomsin the molecule will have the linear translational movement regardlessof the heating method (including microwaves). However, during microwaveexposure there is the additional rotational movement of the polar watermolecule. The same applies to numerous other polar groups or moleculesin any solvent, reagent or reactant, which have densities close to thatof water.

One goal of the invention is to incorporate the solvent or reagent vaporor liquid by infiltration into a film in a carefully controlled mannerjust prior to microwave treatment, so that while the polar vapor orreactant molecules are still available within the pore volume,microwaves will interact with these polar groups or molecules andenhance their mobility and therefore enhance the reaction. The samereaction is expected with higher temperature (by other heating means)but only due to the linear translational movement introduced at thosetemperatures. With microwave heating at the same temperature, the lineartranslational movement is still there, however the additional rotationalmovement of the polar molecules is what enhances the reaction, which inthis case is right within the pore volume. During the microwave heatingprocess the solvent or reagent will be depleted. In some cases, it maybe advantageous to let the sample cool, which will be much faster sincemicrowave heating is a cold wall process, evacuate the chamber, delivermore polar material into the chamber so that fresh liquid or vapors willinfiltrate into the pores, and repeat the microwave heating process.Thus the infiltration and microwave heating could be alternately cycledto allow maximum extent of the reaction desired.

Table I lists some non-polar solvents, which have very small dipolemoments as well as dielectric constants. The non-polar solvent moleculeswill not respond well to microwaves.

TABLE I Properties of some non-polar solvents Non-polar ChemicalDielectric Dipole Boiling Solvents Formula Constant Moment Point, ° C.Pentane CH₃—CH₂—CH₂—CH₂—CH₃ 1.84 0.00 D 36 Cyclopentane C₅H₁₀ 1.97 0.00D 40 Hexane CH₃—CH₂—CH₂—CH₂—CH₂—CH₃ 1.88 0.00 D 69 Cyclohexane C₆H₁₂2.02 0.00 D 81 Benzene C₆H₆ 2.3 0.00 D 80 Toluene C₆H₅—CH₃ 2.38 0.36 D111 1,4-Dioxane /—CH₂—CH₂—O—CH₂—CH₂—O—\ 2.3 0.45 D 101 Chloroform CHCl₃4.81 1.04 D 61 Diethyl ether CH₃CH₂—O—CH₂—CH₃ 4.3 1.15 D 35

In contrast, the polar solvents listed in Table II have significantlyhigher dielectric constants and dipole moments. Like the watermolecules, in presence of microwave energy these polar molecules will beset into rotational movement (possible in available space). Anywhere thevapors of these solvents can deposit, even deep into the pores of theporous dielectric film, microwave energy has the capability to agitatethese molecules and stir up the reaction. It will be preferred to staybelow the boiling point of the solvent or reagent to allow someadditional rotational movement within the pores before going to higherprocess temperature.

TABLE II Properties of some polar solvents Polar Chemical DielectricDipole Boiling Solvents Formula Constant Moment Point, ° C. Water H—O—H80 1.85 D 100 Ethanol CH₃—CH₂—OH 24.5 1.69 D 79 Methanol CH₃—OH 33 1.70D 65 Isopropanol (IPA) CH₃—CH(—OH)—CH₃ 18 1.66 D 82 Acetic acidCH₃—C(═O)OH 6.2 1.74 D 118 Acetone CH₃—C(═O)—CH₃ 21 2.88 D 56 n-PropanolCH₃—CH₂—CH₂—OH 20 1.68 D 97 n-Butanol CH₃—CH₂—CH₂—CH₂—OH 18 1.63 D 118Formic acid H—C(═O)OH 58 1.41 D 101 Propylene C₄H₆O₃ 64.0  4.9 D 240carbonate Ethyl acetate CH₃—C(═O)—O—CH₂—CH₃ 6.02 1.78 D 77 Dimethylsulfoxide CH₃—S(═O)—CH₃ 46.7 3.96 D 189 Acetonitrile (MeCN) CH₃—C≡N 37.53.92 D 82 Dimethylformamide H—C(═O)N(CH₃)₂ 38 3.82 D 153 Tetrahydrofuran/—CH₂—CH₂—O—CH₂—CH₂—\ 7.5 1.75 D 66 Dichloromethane CH₂Cl₂ 9.1 1.60 D 40

FIG. 1 illustrates the schematic details of a microwave apparatus thatwill allow infiltration of the solvent or reagent into the porousdielectric film and in-situ microwave treatment of film on substrates,performing the method disclosed in this invention. The apparatusconsists of a microwave chamber 10, powered by single or multiplemicrowave sources 11. The substrates with porous dielectric film can beloaded into the chamber by lowering a platform 15 mounted on an elevator(not shown), like many of the state of art batch wafer chambers. Withinthe microwave chamber is a quartz bell jar 20, into which the solventsor vapors to be infiltrated are delivered. The substrate 21 is placed ona dielectric/susceptor/cooling stage assembly 22 (with vacuum suction)and rotated slowly to evenly distribute the vapors over the film on thesubstrates. The dielectric assembly or cooling stage 22 may include aconventional lift pin mechanism as commonly used in the industry.

The vapors of the solvent or reagents to be infiltrated into the porousfilm can be brought into the chamber through the tubing 23 and deliveredon the substrate by the shower head, atomizer, diffuser, or other spraymechanism 24. It may be desirable to have just enough fluid dispensedfrom the shower head to form a thin liquid layer on the porousdielectric film to allow the fluid to fill the pores. With more fluid inthe pore contacting the entire inner surface of the pore the rotation ofthe polar molecules will perform the removal process more efficiently.Depending on the application, one might use a single polar materialsource 30 or a bank of sources containing different materials controlledand adjusted through control valves 31. Since multiple sources of vaporscan be used with this technique, the temperature of each of thesesources of vapors could be adjusted to get to the temperature forappropriate vapor pressure. An inert carrier gas 32 carries the vaporsto shower head 24 in the quartz bell jar 20. Although the inert gasshown in the FIG. 1 is N₂, others inert gases (argon, H₂, and forminggas) could be used as well.

A vacuum pump 33 can be used to evacuate the chamber to remove theeffluents or gaseous by-products. The vacuum pump and intake vapor feedcould be adjusted to maintain any desired vapor pressure for theprocess. Preferably, pumping will not be done not be on during themicrowave heat treatment, because at low pressures (<100 Torr) there isthe possibility of generating plasma. Microwave plasma processing isperformed routinely and can be performed here as well, for example forcleaning of the chamber but it is not the main emphasis of thisinvention. What is important is the application of microwaves to acoated substrate having a carefully controlled amount of a polar solventthat will interact with the applied microwave power.

FIG. 2 illustrates the schematically how the pressure, vaporinfiltration, microwave power and temperature cycling dynamics might becarried out for the case where a workpiece is subjected to multiplecycles of infiltration and heat treatment in order to achieve a desiredamount of processing. After the substrate with porous dielectric filmhas been placed in the chamber, FIG. 1, on stage 22 and the system door15 has been closed, the first step is to evacuate the bell jar 20 downto mTorr range to draw the air out of the pores in the porous dielectricfilm. This is shown as pressure and vacuum pump down 100 in FIG. 2. Atthis point, the appropriate vapors of the solvent or reagent can bedelivered into the quartz chamber and allow the pressure to climb up tofew hundred (>200) Torr so that there is no chance of generating aplasma when microwave power is introduced. At this pressure, the poresevacuated down to mTorr range will be infiltrated with the solvent orreagent vapors. Now, it is time to turn microwave power on (shown as 200in FIG. 2) and let microwaves interact and rotates the polar moleculesof the solvent deep in the pores of the porous dielectric film. Theserotating agitators can assist in enhancing the desired process. It wouldbe preferred that the temperature (shown as 300 in FIG. 2) of thesubstrate is maintained for a selected time (e.g., 1-2 minutes) belowthe boiling point of the solvent to avoid removing the solvent sorapidly that it won't have time to promote or enhance the desiredreaction. The temperature can then be increased to the final processtemperature and held for a second time, e.g., ˜5 minutes, when themicrowave power can be turned off and the substrate can be allowed tocool to a temperature lower than the boiling point of the solvent. Theentire cycle can be repeated again by evacuated the chamber and thepores, infiltrating them with fresh solvent or reagent and continue thepolar molecule agitation within the pores followed with the heattreatment. Since everything in the chamber is cool except the substrateand the porous film this cyclic process is easily possible withmicrowave and it is only microwaves that can selectively provide theagitation of the polar molecules within the pores.

It will be understood that if liquid solvent is dispensed from a showerhead, atomizer, or the like, the pressure in the chamber will generallybe ambient pressure as opposed to a reduced pressure or partial vacuum.

The two primary processes addressed in this invention that can beconducted by the above method and apparatus are now described in moredetail.

Solvent Infiltration and porogen removal: The preparation of porous OSGfilms with PECVD and spin-on self-assembled technology to formnano-composite structures was briefly discussed above in the BACKGROUNDsection. PECVD has been a popular approach for many processes in thesemiconductor manufacturing and has been useful for depositing filmswith a higher dielectric constant. For low-k films, a sacrificialporogen is added into the gas stream which on heat treatment is removedfrom the film and thus the pores are formed within the film. However,PECVD films are not able to satisfy all the requirements because of theuncontrollable process of pore formation and some porogen residues leftduring cure that increase the leakage current and decrease the breakdownvoltage.

The spin-on films with self-assembled porous structures are underinvestigation and the advantage of these porous structures are that theyare formed without the use of a sacrificial porogen so there is no riskof residues during cure. These low-k films are formed by condensing ahydrolyzed alkylated silica sol in the presence of a polymericsurfactant. This surfactant acts as a template to produce a regularporous structure as the film dries. However, a cure process is stillnecessary so that the surfactant which acts as a porogen is completelyeliminated, thereby leaving behind a porous silica network with alkylgroups (e.g. methyl —CH₃), which can passivate the internal pore andexternal film surfaces.

The cure process can be conducted using UV, conventional thermal oven,or microwave energy. The surfactants are generally soluble in water orsolvents, so one aspect of this invention is to infiltrate water orsolvent into the pores of the film and expose the film on the substrateto microwaves. This exposure of microwaves to the nano-scale polar wateror solvent molecules within the pore of the film will make them act asnano-agitators and during the rotation or spinning movement within thepores will assist in surfactant or porogen removal from the surface ofthe pores.

The description and data from such experiments on the spin-on dielectricfilms infiltrated to solvent and then exposed to microwaves is givenbelow in the Examples.

Restoration of plasma damaged surface: The other issue to be resolvedwith these dielectric films is the restoration of the surface damagecaused by the use of plasma for etching and cleaning purposes.

During the semiconductor device fabrication processes there are numerousetching/ashing/cleaning plasma processes. Due to the weak strength theporous low-k dielectrics are susceptible to plasma damage. The plasmaexposure breaks the weakly bonded organic terminal groups from thesilica back bone and simultaneously densifies the porous medium furtherto form a silica-like damaged skin layer. This damage and aggravatedsurface roughness degrades the dielectric and electrical properties ofthe devices.

For restoring this damage a variety of materials are being exploredwhich include but are not limited to trialkyl-substituted disilazanes orchlorosilanes such as hexamethyldisilane (HMDS) andtrimethylchlorosilane (TMCS) or dimethylaminotrimethylsilane (DMATMS),phenyldimethylchlorosilane PDMCS, diphenyltetramethyldisilazane DPTMDSand so on. HMDS is generally useful for conditioning chromatographycolumns and for deactivating glassware or silica surface. For plasmadamage restoration process HMDS will restore the pore surface byreplacing the hydroxyl (—OH) group with methyl (—CH₃) group according tothe following reaction.

2(≡Si—OH)+(CH₃)₃Si—NH—Si(CH₃)₃→2(≡Si—O—Si—(CH₃)₃)+NH₃

As described above, HMDS is very popular chemical reagent for enhancingsynthesis reaction with microwaves. Thus one aspect of this invention isto infiltrate HMDS into the pores of the dielectric material and exposeit to microwave energy. Any polar groups (—OH) or molecules within thepores of the dielectric will be set into rotation thereby providingagitation of HMDS molecules to assist in the restoration of the entiresilica surface area damaged with plasma processes. The only reactionbyproduct is ammonia, which happens to be a polar molecule, too. Thusthe polar ammonia molecules will also be set into rotation therebyproviding even more agitation of remaining HMDS molecules to assist inthe restoration of the entire surface area damaged with plasmaprocesses. Anywhere these molecules can infiltrate microwave has theability to agitate them deep within the pores and assist in enhancingthe reaction. It would be advantageous to stay below the boiling point(125° C.) of HMDS so that it does not evaporate immediately and has thesufficient interaction with the pore surface before going higher intemperature as shown in FIG. 2 to densify and strengthen the surfacerestored so far. The substrate can then be allowed to cool down to belowthe boiling point of HMDS and another cycle be performed to increase theextent of restoration. This process can be continued for a number ofcycles to make sure restoration is complete. Although only the detailsfor HMDS have been described, similar mechanisms will be in play forother reagents TMCS or DMATMS and so on, listed above.

Following are a few examples that illustrate the benefit that providethe basis for this invention.

EXAMPLE

Cure and porogen removal of low-k dielectric films: Various materialsand methods have been explored for deposition of porous organo-silicateglass (OSG) low-k films during the last 10-15 years. Spin-on materialswith self-assembled porous structures have also been investigated. Likemany dielectrics, these films are soft baked at 150° C. for 2 minutesand are then finally cured at 450° C. for 30 minutes. These spin-onfilms have been cured with VFM under various conditions and the data arepresented in Table III. In runs 1-3 no solvent was used to infiltratethe pores of the porous dielectric film, whereas water was used for run4 and ethanol for run 5. For each case the Refractive Index (RI),Hardness (H) and Young's Modulus (E) were measured after the VFMprocess, Table III.

TABLE III Microwave process results with and without added solventsYoung's Time, Refractive Hardness Modulus Run Solvent T, ° C. min. IndexH (GPa) E (GPa) 1. VFM N/A 350 30 1.246 0.42 3.88 2. VFM N/A 400 251.248 0.47 4.36 3. VFM N/A 400 15 1.255 0.42 3.86 4. VFM + Water 400 101.251 0.43 3.91 Solvent 5. VFM + Ethanol 400 10 1.245 0.45 4.04 Solvent

Comparing runs 3, 4, and 5 shows a very striking result. Run 3 appliedVFM heating only, without the use of any solvent before the final heattreatment at 400° C. for 15 minutes. Samples for run 4 and 5 had beeninfiltrated with water and ethanol, respectively, by briefly dippingthem in the liquid solvents immediately before the final 400° C. VFMheat treatment. As shown in FIG. 2, the first heating step is below theboiling point of the solvent so for run 4 where water (B.P.=100° C.) wasused, the sample was heated to 90° C. for 2 minutes before going to 400°C. where it was heated for 10 minutes. For run 5 where ethanol was used(B.P.=78° C.) the sample was heated to 65° C. for 2 minutes and thenfollowed with 10 minutes at 400° C. For the data presented there wasonly one exposure to the solvent followed by the higher temperature(400° C.) heat treatment.

Comparing the data for runs 3 and 4 one observes that even though therun 3 had 15 minutes at 400° C., the refractive index for run 4 (water,400° C., 10 min) is lower (lower the better), while both H and E arehigher. For run 5 (ethanol, 400° C., 10 min) RI was even lower and H andE are even higher. Note that the time for (solvent) runs 4 and 5 is 10minutes as compared to 15 minutes for run 3 (no solvent).

It is important to note that the result of this experiment iscounter-intuitive. Recalling that the as-made film had been soft-bakedto remove most or all of the solvent, one would expect that by addingsome solvent back into the coating, the following heat treatment mightin fact take longer because of the need to remove that added solvent.Applicants have found, instead, that the added solvent has enhanced theeffectiveness of the microwave process and thereby actually reduced theoverall thermal budget while achieving comparable results.

In the foregoing experiments, the solvent was applied simply by dippingthe film in liquid solvent and removing excess liquid before heattreatment. It will be understood that various means of adding controlledamounts of solvent may be used within the spirit of the invention. Ashower head, spray nozzle, mister, atomizer, or vapor source containedwithin the microwave cavity as shown schematically in FIG. 1 may beconvenient for some applications, particularly where various mixtures orcombinations of solvents might be needed, and also where multiple cyclesare anticipated. Alternatively, the soft baked coating may be held in acontrolled humidity or partial pressure of the selected polar solventand then transferred to the microwave cavity for immediate processing asdescribed in the following example. This approach might be particularlyuseful when a single heat treatment is needed along with very preciselevel of control of the amount of solvent introduced.

EXAMPLE

FIG. 3 shows one arrangement, in which one or more substrates, havingsoft baked coatings thereon, are held for a selected time to equilibratein a chamber containing a fixed vapor pressure of the selected solvent.This approach may be conveniently incorporated into a cluster toolenvironment, in which a robotic handler adds one soft baked wafer to thehumidity chamber and withdraws another and moves it to the microwavecavity for treatment. The size of the chamber can be selected toproperly buffer the production rate of the line so that each wafer has asufficient residence time in the chamber to equilibrate at the knownpartial pressure.

It will be appreciated that soft baking, followed by equilibration withsolvent vapor at a lower temperature will be much more precise thantrying to soft bake at a particular temperature to produce the targetsolvent concentration directly. It will be further appreciated that byusing this approach, one could enjoy more process flexibility because afirst solvent might be used to form the coating, whereas a secondsolvent may then be introduced after soft baking, with the secondsolvent being more desirable for a particular microwave treatment.

EXAMPLE

There are several ways of establishing a known vapor pressure of solventin a closed chamber. One could construct a humidistat that willautomatically maintain a humidity value using a combination of a heatedor cooled solvent tank and a humidity sensor. Alternatively, it is wellknown that various chemical mixtures can be used, which will have afairly well-known equilibrium vapor pressure in the headspace over them.For example, saturated aqueous solutions of various salts, held atselected temperatures, may be used to generate a relative humidity from9 to 99.7%; similarly, aqueous solutions of sulfuric acid may be used toproduce a relative humidity ranging from 100% to as low as 3.2%depending on the strength of the acid solution [see, e.g., CRC Handbookof Chemistry and Physics, 60^(th) Edition, p. E-46 (1979)].

EXAMPLE

The experimental results shown herein demonstrate that even with asingle exposure to solvent the film properties improved over the VFMheating alone, demonstrating the advantage of being able to agitate thepolar molecules within the pores of the porous dielectric film. Asmentioned above, the dielectric films are generally hydrophobic innature; still the polar molecules of water were able to improve finalproperties of the film. When ethanol is used as the solvent theproperties improve even further. It will be appreciated that one couldadd more cycles to have multiple exposures to the solvent as shown inFIG. 2 if needed to achieve optimal properties in a particularapplication.

Multiple heating cycles may better ensure the mechanical properties ofthe porous dielectric film are robust enough to handle the subsequentsemiconductor device fabrication processes. However, if there is someplasma damage occurring during the numerous etching/ashing/cleaningplasma processes, the following example illustrates how to repair thatdamage.

EXAMPLE

Restoration of the plasma damaged porous dielectric films: In theexample above, the VFM agitation of the polar solvent molecules withinthe pores of the film was demonstrated to improve the film properties.Although these films can be formed with lower k values, successfulimplementation of these porous dielectrics structures poses numerouschallenges including their susceptibility to damage induced duringetching and cleaning process. Much effort is being dedicated to minimizethe plasma damage in the first place, while post-patterning remedies fordielectric restoration are being considered as a second approach.

The first example demonstrated a process which has the potential tominimize the plasma damage whereas this example addresses therestoration of the plasma damage. HMDS is well known for conditioningand deactivating silica surface by replacing the hydroxyl (—OH) groupwith methyl (—CH₃) group according to the reaction shown above.

HMDS by itself has low polarity and does not interact very well withmicrowaves, yet it has been explored as a good reagent to speed up othermicrowave synthesis reactions. The purpose here is not to exploit itspolarity but to use its capability to restore silica surfaces,infiltrate the vapors into the pores of the porous dielectric structureand use microwaves to speed up the reaction that it already is capableof performing by interacting with the polar hydroxyl (—OH) groups. Theonly reaction byproduct—ammonia, is also a polar molecule, and itsagitation with microwave will assist the remaining HMDS molecules in therestoration of the entire surface area damaged with plasma processes.Since the boiling point is 125° C., the temperature should be maintainedbelow that for about a few minutes or so to allow sufficient replacementof hydroxyl (—OH) groups with methyl (—CH₃) groups before going tohigher temperatures as shown in FIG. 2 to densify and strengthen thesurface restored. The substrate can then be allowed to cool down tobelow the boiling point of HMDS and another cycle be performed toincrease the extent of restoration. This process can be continued for anumber of cycles to make sure restoration is complete.

HMDS is used as an example but it will be appreciated that the inventionextends to other reagents which include, but are not limited to,trialkyl-substituted disilazanes or chlorosilanes such ashexamethyldisilane (HMDS) and trimethylchlorosilane (TMCS) ordimethylaminotrimethylsilane (DMATMS), phenyldimethylchlorosilane PDMCS,diphenyltetramethyldisilazane DPTMDS and so on.

The above examples discuss some preferred means for delivering solventor reagent vapors, alone or in combination, to infiltrate porousdielectric films and assist in completing a desired reaction, followedby an in-situ heat treatment to completely cure or densify thedielectric film.

It will be appreciated that although some of the foregoing examples anddiscussion were presented with particular reference to VFM systems usinga TWT amplifier as taught in several of the cited references, theinvention includes any and all systems that may employ microwavegenerators using single or multiple magnetrons, klystrons, gyrotrons, orother microwave power generating devices as are well known in the art.The applicator cavity will generally be multimode for larger substrates,but for smaller specimens a single-mode applicator as is well known inthe art may be also be used.

I claim:
 1. An apparatus for thermal treatment of dielectric films on substrates comprising: a microwave applicator cavity for heating a workpiece, said workpiece comprising a porous coating on a selected substrate; a microwave power source to deliver power to said applicator cavity; and, a means of introducing a controlled amount of a polar solvent into said porous coating immediately before heating by said microwave power.
 2. The apparatus of claim 1 wherein said selected substrate comprises a semiconductor wafer and said porous coating is selected from the group consisting of: low-k dielectric films, organo-silicate glass films, and soft-baked polymeric films.
 3. The apparatus of claim 1 wherein said polar solvent is selected from the group consisting of: water, methanol, ethanol, isopropanol, acetic acid, acetone, n-propanol, n-butanol, formic acid, propylene carbonate, ethyl acetate, dimethyl sulfoxide, acetonitrile, dimethylformamide, tetrahydrofuran, and dichloromethane.
 4. The apparatus of claim 1 wherein said means of introducing said polar solvent comprises a dispenser located within said microwave cavity and positioned above the surface of said porous coating.
 5. The apparatus of claim 4 wherein said dispenser comprises a diffuser dispensing a controlled flow of solvent vapor in a selected carrier gas.
 6. The apparatus of claim 4 wherein said dispenser discharges said solvent in liquid form onto said porous coating, and said dispenser is selected from the group consisting of: shower heads, misters, spray nozzles, and atomizers.
 7. The apparatus of claim 1 wherein said means of introducing said polar solvent comprises a separate chamber in which said porous coating may equilibrate with a controlled vapor pressure of said solvent before placement of said workpiece into said microwave applicator cavity.
 8. The apparatus of claim 7 further comprising a robotic handler to transfer soft-baked workpieces into said chamber and transfer equilibrated workpieces from said chamber to said microwave applicator cavity.
 9. The apparatus of claim 1 wherein: said applicator cavity is selected from the group consisting of: single-mode and multimode cavities; and, said microwave source is selected from the group consisting of: TWT amplifiers, magnetrons, klystrons, and gyrotrons.
 10. An apparatus for thermal treatment of dielectric films on substrates comprising: a microwave applicator cavity for applying microwave power to a workpiece comprising a porous coating on a selected substrate, said applicator cavity having therein: a supporting structure upon which said workpiece may be placed with said porous coating facing upward, and, a dispenser positioned above said coating to dispense a controlled amount of a selected polar solvent onto said coating prior to the application of microwave power; and, a microwave power source to supply power to said applicator cavity.
 11. The apparatus of claim 10 wherein said polar solvent is selected from the group consisting of: water, methanol, ethanol, isopropanol, acetic acid, acetone, n-propanol, n-butanol, formic acid, propylene carbonate, ethyl acetate, dimethyl sulfoxide, acetonitrile, dimethylformamide, tetrahydrofuran, and dichloromethane.
 12. The apparatus of claim 10 wherein said dispenser comprises a diffuser dispensing a controlled flow of solvent vapor in a selected carrier gas.
 13. The apparatus of claim 10 wherein said dispenser discharges said solvent in liquid form onto said porous coating, and said dispenser is selected from the group consisting of: shower heads, misters, spray nozzles, and atomizers.
 14. An apparatus for thermal treatment of dielectric films on substrates comprising: a pretreatment chamber for receiving a soft-baked workpiece, said workpiece comprising a porous coating on a selected substrate, and holding said workpiece in a controlled vapor pressure of a selected polar solvent for a sufficient time to allow a selected amount of said solvent to be absorbed by said porous coating; a microwave applicator cavity for applying microwave power to said workpiece so that a portion of said microwave power interacts with said absorbed solvent; a microwave power source to supply power to said applicator cavity; and, a material handler to place said soft-baked workpiece into said pretreatment chamber, remove a pretreated workpiece from said chamber, and place said pretreated workpiece into said applicator cavity for microwave processing.
 15. The apparatus of claim 14 wherein said selected substrate comprises a semiconductor wafer and said porous coating is selected from the group consisting of: low-k dielectric films, organo-silicate glass films, and soft-baked polymeric films.
 16. The apparatus of claim 14 wherein said polar solvent is selected from the group consisting of: water, methanol, ethanol, isopropanol, acetic acid, acetone, n-propanol, n-butanol, formic acid, propylene carbonate, ethyl acetate, dimethyl sulfoxide, acetonitrile, dimethylformamide, tetrahydrofuran, and dichloromethane.
 17. The apparatus of claim 14 wherein said pretreatment chamber comprises a humidistat.
 18. The apparatus of claim 14 wherein said pretreatment chamber contains a reservoir of said polar solvent further containing a dissolved species so that a selected partial pressure of said solvent is maintained in a headspace above said reservoir.
 19. The apparatus of claim 18 wherein: said polar solvent is water; said dissolved species comprises one or more compounds selected from the group consisting of: acids and salts; and, said dissolved species is selected to maintain a relative humidity from about 3.2% to about 100% in said headspace.
 20. An apparatus for thermal treatment of dielectric films on substrates comprising: a microwave applicator cavity for heating a workpiece, said workpiece comprising a porous organo-silicate glass (OSG) low-k film on a selected substrate; a microwave power source to deliver power to said applicator cavity; and, a means of introducing a controlled amount of a surface conditioning reagent into said porous OSG coating immediately before heating by said microwave power, wherein said surface conditioning reagent is selected from the group consisting of: trialkyl-substituted disilazanes, chlorosilanes, hexamethyldisilane (HMDS), trimethylchlorosilane (TMCS), dimethylaminotrimethylsilane (DMATMS), phenyldimethylchlorosilane (PDMCS), diphenyltetramethyldisilazane (DPTMDS).
 21. The apparatus of claim 20 wherein said surface conditioning reagent is further combined with a polar solvent. 